You have to use the SPAS as your secondary weapon. When you switch to SPAS, your reload button will change from the default function (tap to reload, or hold to keep reloading SPAS shells), into a toggle on/off. So you tap R, and reload goes into a loop to keep loading shells. Reload toggles off again when you tap R, switch to a new weapon, or drop the SPAS.

In other words, you don't have to keep a finger on reload to keep loading shells until the SPAS is full.